To make the dressing:Completely dissolve sugar in water; stir in fish sauce, lime juice and sriracha.
To prepare the beef:Combine oil, lemongrass, fish sauce, soy sauce and sugar; stir until sugar dissolves. Add beef and coat thoroughly with marinade. Marinate at room temperature for 20 minutes. Grill beef over medium-hot heat until desired doneness is achieved; allow to rest before slicing, against the grain, into thin strips.
To assemble and serve:Transfer grilled meat to a large bowl. Add mango, red onion and dressing; toss gently. Add cilantro and basil; toss gently again before plating. Garnish with peanuts.
Source: Chef Ben Randolph, The Broadway Hotel, Columbia, MO